In the DE OS No. 2 755 575 a laser-operated signal recording apparatus is described that comprises an acousto-optical cell utilizing Bragg diffraction for generating a plurality of simultaneously outgoing diffracted laser beams. The outgoing laser beams are projected on a signal recording surface by means of a suitable optical system and the beams are also deflected to scan a band-like raster on the signal recording surface. The acousto-optical cell is controlled in accordance with the information defined by the shape of the characters to be recorded, and the scanning beams illuminate the recording surface in locations that correspond to the presence of the characters. The areas that do not comprise characters are not illuminated at all.
This known apparatus provides for rapid information recording, and because of to the application of a multi-beam raster the speed of its electronic circuitry does not have to be above normal values.
There are a large number of conventional multi-beam information recording devices including the above cited system that have a common drawback, namely, that the recording surface is illuminated at locations corresponding to the positions of the characters to be recorded. Usually in the recorded documents the character locations should have a definite shade that distinguishes them from the generally white shade of the carrier (paper). For ensuring such properties in conventional laser operated recording equipment of the above type, an electrophotographic image reversal is used, by means of which the recorded document comprises positive (black) surfaces at the illuminated locations.
The moodes used for electrophotographic image reversal are substantially complex and expensive compared to the widely used document copying equipments intended for use in offices. In such equipments positive copies are produced without electrophotographic image reversal, and the copy is black at locations in which the original document is black.
In order to eliminate the latter type of image reversal in laser operated recording equipment the image reversal may be carried out by optoelectronic means by the appropriate control of the laser beam used for recording the characters.
In the periodical "Electronics" (No. 5 1979, pp 46 to 48) an apparatus is described, in which a single laser beam scans a raster on the recording surface, and the control of the laser beam occurs in accordance with optoelectronic image reversal. This way in that apparatus there is no need for electrophotographic image reversal, and positive copier units can be used for the image recording.
The application of a single laser beam that scans each raster line of the character separately necessitates a remarkable increase in the scanning frequency compared to multi-beam recording technique, if the time of the recording remains unchanged. The required fast deflection of the laser beam cannot be effected by means of the widely used swinging mirror technique. Such fast scanning can be realized by an expensive rotating mirror or by means of an additional acousto-optical cell. Due to the higher speed used in the single beam technique, the speed of controlling the acousto-optical cell must also be increased in a corresponding manner. This means that high speed electronic circuits must be used, which substantially contributes to the production costs. To ensure an undistorted image reconstruction from the raster image a precise synchronism should be provided between the movement of the scanning beam and the displacement of the recording medium. Additionally, a synchronism between subsequent raster lines should be provided with an accuracy in the order of a few nanoseconds.
Because of these technical difficulties connected with this approach the advantages deriving from the elimination of electrophotographic image reversal are at least in great part be compensated by the problems connected with the optoelectronic image reversal when a single raster beam is used.